We all know that Robert Frank is the author of Cokcsucker Blues which remains officially unreleased, but what about "S-8 Stones Footage from Exile on Main St." which is available on Frank's The Complete Film Work Vol. 3 ?
Does anyone have watched it and can tell something about it ?

At [www.steidl.de] , they just say "Filmed during the making of the Exile on Main St., Rolling Stones album cover."

I've found this on the web : [microcinema.com]

Appaently it's a short movie with no sound...

Yeah it's a very short film... and probably the only Stones-related material Frank was allowed to put on his retrospective DVD set before Jagger unleashed his lawyers on him.
In other words CSBlues was announced as part of the retrospective DVD back in 2010 but Jagger made sure it didn't happen.

So we're left with hours and hours of artsy-fartsy 16mm grainy B&W films made by Frank when he quit photography...

There was a screening in NY 2 years ago: [www.moma.org]

By the way, does anyone know where to find the original photograph from Frank's book, from which the Exile cover was created?

It's from "The Americans". It's reissued every other year so it's easily available.